The invention relates to power conversion electronics.
Alternating current (AC) has been almost universally adopted for utility power distribution and consequently is the standard form of electrical power for industrial, commercial, and domestic purposes. Independent of the source of energy used to generate the electricity (for example, hydroelectric, nuclear, solar, wind-power), AC must be provided at a fixed frequency of 60 Hz in North America (usually 50 Hz elsewhere) and phase-synchronized before being made available to the large power grid from which users obtain electricity.
Semiconductor-based power electronic converters are often used to conform electrical power generated from various power sources to the 60 Hz fixed frequency, phase-synchronized AC required by the grid.
Power compensation systems are often used to provide real and/or reactive power to a utility power system in response to voltage instabilities or fault conditions on the systems. In such power compensation systems, DC sources including batteries, capacitor banks, fuel cells, or superconducting magnetic energy storage (SMES) devices supply power to an inverter which converts the DC to AC before introduction to the utility grid. Unfortunately, the DC-AC rectification/inversion process wastes a portion of the generated power due primarily to the dissipation occurring within the large energy storage devices (e.g., inductors) and within the semiconductor devices themselves.
DC to AC power converters typically incorporate switching circuitry which receives a DC voltage and is controlled to generate a pulse width modulated (PWM) signal. This power signal is then provided to a filter network to provide an AC power signal. Typically, the DC signal is pulsed and applied to the primary windings of a transformer. This, in turn, generates a pulsed signal on the secondary windings of the transformer, where the amplitude of the secondary signal is varied in accordance with the ratio of primary to secondary transformer windings. A rectifier and capacitor are typically employed to smooth the pulsed secondary voltage into a AC voltage.
DC to DC converters are often used to convert a DC signal of a first amplitude into a DC signal of a second amplitude. One type of DC to DC converter is known as a xe2x80x9cbuckxe2x80x9d converter and uses a switching device to pulse the DC power signal across a frequency dependant filter network, such as an inductive-capacitive (or LC) filter. The amplitude of the signal is directly proportional to the duty cycle of the pulsating signal driving the switching device. Typically, these switching devices are power transistors, relays, or any other form of electronic switching device.
The invention relates to power conversion circuitry operated to perform DC-AC, DC-DC, AC-DC, and AC-AC power conversion.
In one aspect of the invention, an integrated assembly includes a power converter module having an input bus bar, an output bus bar, an n-level inverter and driver circuitry adapted to control the n-level converter, in response to received control signals. The n-level converter switches between a pair of voltage levels selected from a set of n levels, where n is 3 or greater. The integrated assembly also includes a controller providing the control signals to the driver circuitry, and fiber optic lines connecting the driver circuitry of the n-level converter to the controller.
Embodiments of this aspect of the invention may include one or more of the following features. The n-level converter is capable of generating power levels in excess of 1 megawatt and preferably as high as 2.5 megawatts at 7.6 KV. The integrated assembly has a width of approximately 20 inches, a length of approximately 28 inches, and a height of approximately 23 inches. The n-level converter includes a printed circuit board (PCB). The n-level converter is an n-level inverter.
The integrated assembly includes a DC-DC converter which, in operation, receives a DC input voltage, generates a DC output voltage and provides the DC output voltage to the n-level inverter. The DC-DC converter includes controlled switching devices (e.g., power transistors), each receiving a portion of the DC input voltage and each having a voltage rating characteristic less than said DC input voltage. The sum of the voltage rating characteristics of each of the controlled switching devices is greater than the DC input voltage. The controlled switching devices include a first switching device and a second switching device, and the DC-DC converter includes a filter circuit connected between output terminals of the first and second switching devices. The filter circuit includes a capacitive device for providing the DC output voltage to a load, such as auxiliary electronic circuitry associated with the n-level converter. The integrated assembly includes a diode, positioned between the outputs of the first and second switching devices, for providing a discharge path for the capacitive device of the filter circuit.
The controller is adapted to selectively energize and deenergize said switching devices, a duty cycle of the switching devices controlling the amplitude of the DC output voltage. The controller, in operation, is configured to monitor the DC output voltage and adjust said duty cycle of the switching devices to maintain said DC output voltage at a predetermined level.
The controller includes protection circuitry, which in response to an indication of a fault condition of the integrated assembly provides a signal to the controller to terminate operation of the n-level power converter module. The fault condition may be an overvoltage, undervoltage, overcurrent, or an over-temperature condition. The protection circuitry includes a sensor, which monitors the output of the n-level power converter module and, in response to an overvoltage condition at an output of the n-level power converter module, provides the signal to the controller to terminate operation of the n-level power converter module. The sensor monitors the output current of the n-level power converter module. The integrated assembly also includes a cooling system including, for example, a heat sink.
Among other advantages, the n-level power converter module is used in a stand-alone configuration, integrated, for example, with a high power DC power source. In addition, the microcontroller for providing the intelligence required by the n-level converter module is part of (i.e., on-board) the integrated assembly. Because the xe2x80x9con-boardxe2x80x9d microcontroller can be programmed, the n-level converter module""s functionality can be changed for use in different applications. Furthermore, the n-level inverter module is bi-directional. By xe2x80x9cbi-directionalxe2x80x9d it is meant that electric power is allowed to flow in either direction through the n-level inverter. The power flowing out of the inverter can have different characteristics than the power flowing in; providing a method for conditioning the power. Thus, the microcontroller of the n-level inverter module can be programmed to perform AC-DC conversion (rectification), DC-DC conversion, DC-AC conversion (inversion), and AC-AC conversion. For example, in one application, the integrated assembly is used to condition power for a motor drive, while in another application, it is used as part of an uninterruptible power supply (UPS). The ability to use the same integrated assembly for different applications provides tremendous flexibility to the user. Although the various parts of the system (e.g., protection circuitry, switch sequencing) can operate relatively autonomously, the particular manner in which they operate can be changed to, suit a particular application.
Fiber optic lines provide high speed, noise immune communication of signals between components of the system; thus, transmission losses are reduced. Furthermore, because the n-level converter is constructed on a printed circuit board, the interconnection paths between components (e.g., high power switching devices) of the converter and drive circuitry are reduced. In essence, the interconnection paths designed within the PCB replace many of the relatively long interconnection paths typically used to interconnect components.
The components of the n-level inverter module are assembled together in a hybrid assembly including bus bars and PCBs, thereby reducing size and cost. The hybrid assembly also eliminates much of the wiring typically associated with conventional high power assemblies. In particular, automated wave-soldering, short interconnects, and direct connections are used to interconnect the components in the hybrid assembly. With this arrangement, the only external buswork required is that between the integrated assembly and the outside world to which it supplies power. Thus, a xe2x80x9cconnect-and-goxe2x80x9d integrated assembly of reduced modular size that is easy to fabricate is provided. And aside from all of the advantages of modularizing the assembly, by providing the interconnects using a non-inductive approach, the electrical performance of the assembly is significantly improved.
In general, minimizing parasitic losses in the assembly reduces the overall losses of the unit. Thus, the integrated approach allows the use of smaller, generally more available and less expensive components and reduces cooling requirements.
Among the interconnection paths being replaced are the relatively bulky copper buses having large screw terminals and separate snubber capacitors having leads connected to the buses. These fabricated busworks and bulky interconnects contribute substantially to the stray inductance and capacitance in the assembly. These parasitic losses tremendously limit the available power throughput of the circuitry as well as the speed at which the switching devices in high power inverters can be switched. The hybrid construction provides relatively short interconnection paths between the high-powered switching components thus minimizing inductance and other parasitic losses. Minimizing these parasitic losses eliminates the need for additional circuitry, such as capacitive snubbers, typically used to compensate for capacitance. Furthermore, the level of filtering at the output of the assembly is reduced, thereby reducing the size and power loss associated with such circuitry.
In another aspect of the invention, a switching power supply includes controlled switching devices, each receiving a portion of a DC input voltage and each having a voltage rating characteristic that is less than the DC input voltage, the sum of the voltage rating characteristics being greater than the DC input voltage.
Embodiments of this aspect of the invention may include one or more of the following features. The controlled switching devices include a first switching device and a second switching device and a filter circuit connected between output terminals of the first and second switching devices. The filter circuit includes a capacitive device for providing the DC output voltage to a load (e.g., auxiliary circuitry associated with the n-level inverter discussed above). The first and second switching devices are power transistors. The switching power supply includes at least one diode, positioned between the outputs of the first and second switching devices, for providing a discharge path for the capacitive device of the filter circuit. The switching power supply includes a controller for selectively energizing and deenergizing the switching devices, where a duty cycle of the switching devices controls the amplitude of the DC output voltage. The controller, in operation, is configured to monitor the DC output voltage and adjust the duty cycle of the switching devices to maintain the DC output voltage at a predetermined level.
In another aspect of this invention, a switching power supply includes a first and a second converter circuit, each having a first and a second switching device, each of the first and second switching devices receiving a portion of the DC input voltage and having a voltage rating characteristic less than the DC input voltage. The sum of the voltage rating characteristics for the first switching devices is greater than the DC input voltage of the first and second converters.
Embodiments of this aspect of the invention may include one or more of the following features. The first and second converter circuits based on are buck converter designs. Each of the converter circuits includes a filter circuit, having a capacitor for providing a portion of the DC output voltage to the load. The first switching devices are power transistors, while the second switching devices are diodes for providing a discharge path for each capacitor of each filter.
The switching power supply includes a controller for selectively energizing and deenergizing the first switching devices, with a duty cycle of the first switching devices controlling the amplitude of the DC output voltage. The controller, in operation, is configured to monitor the DC output voltage and adjust the duty cycle of the first switching devices to maintain the DC output voltage at a predetermined level.
The advantages of the above aspects of the invention are numerous. In general, the DC-DC converter switching power supply can convert relatively high DC voltage levels to intermediate or low voltage levels. This advantage is particularly important for applications in which DC voltages lower than that being provided by DC power supplies (e.g., capacitor energy storage banks, batteries, or SMES devices) are required. For example, a relatively higher DC voltage provided to the input of an inverter can be converted to a lower DC voltage for auxiliary electronics associated with the inverter. The mirrored arrangement of the DC-DC converter switching power supply is also self-balancing. That is, if one of the converters tries to draw more power, its voltage will automatically be reduced. This self-balancing feature is performed without separate external control.
In general, the switching power supply employs two or more switching devices which distribute the DC input voltage across these devices, eliminating the requirement that the switching devices have a voltage rating equal to or greater than the DC input voltage. As stated above, this invention allows the designer to utilize multiple, inexpensive, low voltage switching devices, as opposed to a single, expensive, high-voltage switching device.
The DC-DC switching power supply described above is well suited for use in providing an output DC voltage to an inverter. In one embodiment, the load to the DC-DC switching power supply is a tri-level inverter. The switching power supply can be fabricated on a printed circuit board and included as part of an integrated assembly including a tri-level inverter, microcontroller, and other electronics.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.